1. Field of the Invention
The present invention relates to an imaging mass spectrometer used to analyze the positions and intensity distributions of trace compounds and also to a method of controlling this spectrometer.
2. Description of the Related Art
A time-of-flight (TOF) mass spectrometer is an instrument for giving a certain amount of energy to ions such that they are accelerated and travel and for finding the mass-to-charge ratios (m/z) of the ions from the times taken for the ions to reach the detector. In the TOF mass spectrometer, the ions are accelerated by a constant pulsed voltage Va. At this time, from the law of conservation of energy, the following Eq. (1) holds.
                                          mv            2                    2                =                  zeV          a                                    (        1        )            where v is the velocity of the ion, m is the mass of the ion, z is the valence number of the ion, and e is the elementary charge.
From Eq. (1), the velocity v of the ion is given by
                    v        =                                            2              ⁢                              zeV                a                                      m                                              (        2        )            
Therefore, the flight time T required for the ion to reach a detector, placed behind at a given distance of L, is given by
                    T        =                              L            v                    =                      L            ⁢                                          m                                  2                  ⁢                                      zeV                    a                                                                                                          (        3        )            
As can be seen from Eq. (3), the flight time T differs according to ink of each ion. TOFMS is an instrument for separating masses employing this principle.
The mass resolution R of a TOF mass spectrometer is defined as follows:
                    R        =                  T                      2            ⁢            Δ            ⁢                                                  ⁢            T                                              (        4        )            where T is the total flight time and ΔT is a peak width.
That is, if the peak width ΔT is made constant and the total flight time T can be lengthened, the mass resolution can be improved.
The simplest ion optical system for performing mass separation is a linear TOFMS in which ions accelerated by an ion source are made to travel linearly. Also, reflectron TOFMS instruments capable of elongating the flight time by placing a reflectron field between an ion source and a detector have enjoyed wide acceptance. In the linear or reflectron type TOFMS, increasing the total flight time T (i.e., increasing the total flight distance) will lead directly to an increase in instrumental size. A multi-pass time-of-flight mass spectrometer has been developed to realize high mass resolution while avoiding an increase in instrumental size (non-patent document 1). This instrument uses four toroidal electric fields each consisting of a combination of a cylindrical electric field and a Matsuda plate. The total flight time T can be lengthened by accomplishing multiple turns in an 8-shaped circulating orbit. In this multi-pass time-of-flight mass spectrometer of non-patent document 1, positions, angles, and the distribution of kinetic energies can be maintained constant during each revolution.
One ion source for TOFMS makes use of a laser desorption/ionization (LDI) method consisting of irradiating a sample applied on a plate or a sample in the form of a solid with laser radiation to ionize the compound to be investigated. In the laser desorption/ionization method, the ionization efficiency is low in many cases depending on subjects of measurement. Therefore, the matrix-assisted laser desorption/ionization (MALDI) method is widely used. In particular, a sample is mixed and dissolved in a matrix (liquid, crystalline compound, metal powder, or the like), which has an absorption band in the used laser light wavelength and promotes ionization, is solidified, and the matrix is irradiated with laser radiation to vaporize or ionize the sample. In recent years, studies of surface-assisted laser desorption/ionization (SALDI) using a plate on which a nanostructured layer is formed to promote ionization have been in progress.
In the initial state of a laser assisted ionization typified by a MALDI method, ions or neutral particles are ejected explosively at speeds comparable to the sonic speed. Therefore, when ions are generated, large energies are distributed at the beginning. To converge this distribution towards the axis of flight, delayed extraction is used in most cases. This method consists of applying a pulsed voltage with a delay of hundreds of nsec since laser irradiation. Adoption of delayed extraction has improved the performance of MALDI-TOFMS greatly.
A technique for obtaining two-dimensional positional information, information about the masses of compounds contained at each position, and information about their abundances using a mass spectrometer is known as imaging mass spectrometry (IMS) (non-patent document 2). In one ionization method, laser desorption ionization typified by a MALDI method is used. In another ionization method known as TOF-SIMS, fast particles are employed.
One method which is based on imaging mass spectrometry (IMS) employing a MALDI technique and which obtains positional information is scanning IMS (non-patent document 2). Another method is a stigmatic IMS (non-patent document 3). In scanning imaging mass spectrometry, a mass spectrum is collected from each location while scanning the irradiation position. Mass spectra are obtained in the same way as in normal mass analysis methods. In the case of the scanning imaging mass spectrometry (IMS), the limit of the positional resolution is about the diameter of the irradiated spot, i.e., on the order of 10 μm.
On the other hand, in a stigmatic imaging mass spectrometry (IMS) technique, each ion is made to reach the detector while maintaining information about the positions at which the ions are ionized within a region irradiated with laser light. Therefore, the diameter of the spot irradiated with the laser light does not limit the positional resolution, unlike scanning IMS. However, mass separation needs to be done while preventing distortion of the positional information. Therefore, TOFMS is utilized in the mass analyzer. Furthermore, the operation is different from general MALDI-TOFMS. In normal MALDI-TOFMS, ions arriving at the detection surface at regular intervals of time are collected by the detection system and a mass spectrum is created. In the case of stigmatic imaging mass spectrometry, however, it is necessary to obtain a projection image. Therefore, acquisition of information about arrival positions on the detection surface is necessary, in addition to execution of time separation. Consequently, a detection system capable of position separation is required, in addition to time separation. Another great difference is that delayed extraction generally used in normal MALDI-TOFMS cannot be used. In delayed extraction, during laser irradiation (i.e., during ionization), ions are made to travel freely in a free space. After a lapse of hundreds of ns, a pulsed voltage is applied to accelerate the ions. As a result, the ions having a distribution of initial velocities can be detected at the same time on the detection surface. In the case of the stigmatic IMS, however, it is important to hold the ionization position information and so it is necessary to extract ions with a high voltage immediately after the laser irradiation. This makes it impossible to observe ions having a distribution of initial velocities at the same time at the detection surface. Consequently, the mass resolution tends to be lower than in scanning IMS. A technique for providing improved mass resolution by employing multi-turn TOFMS in the stigmatic IMS has been also proposed (patent document 1).